Facilitated CO2 transport membrane and method for producing same, and method and apparatus for separating CO2

ABSTRACT

Provided is a facilitated CO 2  transport membrane having an improved CO 2  permeance and an improved CO 2 /H 2  selectivity. The facilitated CO 2  transport membrane includes a separation-functional membrane that includes a hydrophilic polymer gel membrane containing a CO 2  carrier and a CO 2  hydration catalyst. Further preferably, the CO 2  hydration catalyst at least has catalytic activity at a temperature of 100° C. or higher, has a melting point of 200° C. or higher, or is soluble in water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Phase filing under 35 U.S.C. §371 ofInternational Application No. PCT/JP2013/076665 filed on Oct. 1, 2013,and which claims priority to Japanese Patent Application No. 2012-220250filed on Oct. 2, 2012.

TECHNICAL FIELD

The present invention relates to a facilitated CO₂ transport membranethat is used for separating carbon dioxide (CO₂), particularly to afacilitated CO₂ transport membrane that separates carbon dioxideproduced as a by-product in a hydrogen production process or the like ata high selection ratio to hydrogen. The present invention furtherrelates to a method for producing the facilitated CO₂ transportmembrane, and a method and an apparatus for separating CO₂ using thefacilitated CO₂ transport membrane.

BACKGROUND ART

In a hydrogen production process, it is necessary that CO₂ produced as aby-product in the course of producing hydrogen be separated and removedfrom a hydrogen gas.

A chemical absorption method that is used in a decarbonation processesin existing large-scale plants such as hydrogen production plants andammonia production plants requires a huge CO₂ absorption tower and ahuge regeneration tower for a CO₂ absorbing liquid in order to separateCO₂, and in a regeneration step for the CO₂ absorbing liquid, requires alarge amount of steam for heating the CO₂ absorbing liquid to remove CO₂therefrom so that the liquid absorbing CO₂ can be reused, and thereforeenergy is wastefully consumed.

In recent years, as a countermeasure for global warming, natural energythat does not emit CO₂ has been expected to come into wide use, butnatural energy has a significant problem in terms of cost. Thus,attention has been paid to a method called CCS (Carbon dioxide Captureand Storage) in which CO₂ is separated and collected from waste gasesfrom thermal power plants, ironworks and the like, and buried in theground or sea. Currently, even CCS is based on application of thechemical absorption method. In this case, for separating and collectingCO₂ from thermal power plants, not only large-scale CO₂ separationequipment is required, but also a large amount of steam should be fed.

On the other hand, a CO₂ separation and collection process using amembrane separation method is intended to separate a gas by means of adifference in velocity of gases passing through a membrane using apartial pressure difference as driving energy, and is expected as anenergy-saving process because the pressure of a gas to be separated canbe utilized as energy and no phase change is involved.

Gas separation membranes are broadly classified into organic membranesand inorganic membranes in terms of a difference in membrane material.The organic membrane has the advantage of being inexpensive andexcellent in moldability as compared to the inorganic membrane. Theorganic membrane that is used for gas separation is generally a polymermembrane prepared by a phase inversion method, and the mechanism ofseparation is based on a solution-diffusion mechanism in which a gas isseparated by means of a difference in solubility of the gas in themembrane material and diffusion rate of the gas in the membrane.

The solution-diffusion mechanism is based on the concept that a gas isfirst dissolved in the membrane surface of a polymer membrane, and thedissolved molecules diffuse between polymer chains in the polymermembrane. Where for a gas component A, the permeability coefficient isP_(A), the solubility coefficient is S_(A), and the diffusioncoefficient is D_(A), the relational expression: P_(A)=S_(A)×D_(A)holds. The ideal separation factor α_(A/B), is expressed asα_(A/B)=P_(A)/P_(B) by taking the ratio of permeability coefficientsbetween components A and B, and thereforeα_(A/B)=(S_(A)/S_(B))×(D_(A)/D_(B)) holds. Here, S_(A)/S_(B) is referredto as solubility selectivity, and D_(A)/D_(B) is referred to asdiffusivity selectivity.

Since the diffusion coefficient increases as the molecular diameterdecreases, and the contribution of diffusivity selectivity is generallygreater than that of solubility selectivity in gas separation, it isdifficult to allow selective passage of gases having a larger moleculardiameter by suppressing passage of gases having a smaller moleculardiameter among multi-component gases having different moleculardiameters.

Therefore, it is extremely difficult to prepare a CO₂ selectivepermeable membrane that separates, particularly from a mixed gascontaining H₂ and CO₂, CO₂ with high selectivity to H₂ having thesmallest molecular diameter among gas molecules. It is still moredifficult to prepare a CO₂ selective permeable membrane that is capableof being put to practical use in a decarbonation process in a hydrogenproduction plant or the like and that functions at a high temperature of100° C. or higher.

Thus, studies are conducted on a permeable membrane called a facilitatedtransport membrane that allows selective permeation of a gas by afacilitated transport mechanism, in addition to a solution-diffusionmechanism, using a substance called a “carrier” which selectively andreversibly reacts with CO₂ (see, for example, Patent Document 1 below).The facilitated transport mechanism has a structure in which a membranecontains a carrier which selectively reacts with CO₂. In the facilitatedtransport membrane, CO₂ passes not only physically by thesolution-diffusion mechanism but also as a reaction product with thecarrier, so that the permeation rate is accelerated. On the other hand,gases such as N₂ and H₂, which do not react with the carrier, pass onlyby the solution-diffusion mechanism, and therefore the separation factorof CO₂ with respect to these gases is extremely high. Energy generatedduring the reaction of CO₂ with the carrier is utilized as energy forreleasing CO₂ by the carrier, and therefore there is no need to supplyenergy from outside, so that an essentially energy-saving process isprovided.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: International Publication No. WO 2009/093666

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Patent Document 1 proposes a facilitated CO₂ transport membrane having aCO₂ permeance and a CO₂/H₂ selectivity feasible at a high temperaturecondition of 100° C. or higher by using as a carrier a specific alkalimetal salt such as cesium carbonate or rubidium carbonate.

The facilitated CO₂ transport membrane has a higher CO₂ permeation rateas compared to a membrane based on a solution-diffusion mechanism, butthe number of carrier molecules that react with CO₂ molecules becomesless sufficient as the partial pressure of CO₂ increases, and thereforeimprovement is required for accommodating the membrane to carriersaturation even at such a high CO₂ partial pressure.

Further, there are expectations for provision of a facilitated CO₂transport membrane that is applicable at a high temperature of 100° C.or higher and has an improved CO₂ permeance and an improved CO₂/H₂selectivity in a decarbonation step in a hydrogen production process orthe like.

In view of the above-mentioned problems, it is an object of the presentinvention to stably supply a facilitated CO₂ transport membrane havingan improved CO₂ permeance and an improved CO₂/H₂ selectivity.

Means for Solving the Problems

For achieving the above-mentioned object, the present invention providesa facilitated CO₂ transport membrane comprising a separation-functionalmembrane that includes a hydrophilic polymer gel membrane containing aCO₂ carrier and a CO₂ hydration catalyst. It is to be noted that the CO₂hydration catalyst is a catalyst that increases the reaction rate of theCO₂ hydration reaction shown in the following (Chemical Formula 1). Thesymbol “

” in the reaction formulae shown herein indicates that the reaction is areversible reaction.CO₂+H₂O

HCO₃ ⁻+H⁺  (Chemical Formula 1)

The reaction of CO₂ with the CO₂ carrier is expressed by the following(Chemical Formula 2) as an overall reaction formula. It is to be notedthat the (Chemical Formula 2) is based on the assumption that the CO₂carrier is a carbonate. The CO₂ hydration reaction, one of elementaryreactions of the above-mentioned reaction, is an extremely slow reactionunder a catalyst-free condition, and addition of a catalyst acceleratesthe elementary reaction, so that the reaction of CO₂ with the CO₂carrier is accelerated, and as a result, improvement of the permeationrate of CO₂ is expected.CO₂+H₂O+CO₃ ²⁻

2HCO₃ ⁻  (Chemical Formula 2)

Thus, since the facilitated CO₂ transport membrane having theabove-mentioned features contains a CO₂ carrier and a CO₂ hydrationcatalyst in a separation-functional membrane, the reaction of CO₂ withthe CO₂ carrier is accelerated, so that a facilitated CO₂ transportmembrane having an improved CO₂ permeance and an improved CO₂/H₂selectivity can be provided. Further, since the CO₂ hydration catalysteffectively functions even at a high CO₂ partial pressure, the CO₂permeance and CO₂/H₂ selectivity at a high CO₂ partial pressure are eachimproved. Further, since the separation-functional membrane is composedof a gel membrane rather than a liquid membrane or the like, highselective permeability to hydrogen can be stably exhibited even underpressure.

Further, in the facilitated CO₂ transport membrane having theabove-mentioned features, the CO₂ hydration catalyst preferably hascatalytic activity at a temperature of 100° C. or higher. The reactionof CO₂ with the CO₂ carrier is thereby accelerated at a temperature of100° C. or higher, so that a facilitated CO₂ transport membrane havingan improved CO₂ permeance and an improved CO₂/H₂ selectivity can beprovided under such a temperature condition.

Further, in the facilitated CO₂ transport membrane having theabove-mentioned features, the CO₂ hydration catalyst preferably has amelting point of 200° C. or higher, and is preferably soluble in water.

Further, in the facilitated CO₂ transport membrane having theabove-mentioned features, the CO₂ hydration catalyst preferably containsan oxo acid compound, particularly preferably an oxo acid compound of atleast one element selected from group 6 elements, group 14 elements,group 15 elements and group 16 elements.

Further, in the facilitated CO₂ transport membrane having theabove-mentioned features, the CO₂ hydration catalyst preferably containsat least one of a tellurous acid compound, a selenious acid compound, anarsenious acid compound, an orthosilicic acid compound and a molybdicacid compound.

Particularly, when the melting point of the CO₂ hydration catalyst is200° C. or higher, the catalyst can exist in the separation-functionalmembrane while being thermally stable, so that performance of thefacilitated CO₂ transport membrane can be maintained over a long periodof time. Further, when the CO₂ hydration catalyst is soluble in water, ahydrophilic polymer gel membrane containing a CO₂ hydration catalyst canbe easily and stably prepared. When a tellurous acid compound, aselenious acid compound, an arsenious acid compound, an orthosilicicacid compound or a molybdic acid compound is used as the CO₂ hydrationcatalyst, stable improvement of membrane performance can be expectedbecause all of these compounds are water soluble and have a meltingpoint of 200° C. or higher.

Further, in the facilitated CO₂ transport membrane having theabove-mentioned features, the gel membrane is preferably a hydrogel,further preferably a polyvinyl alcohol-polyacrylic acid (PVA/PAA) saltcopolymer gel membrane.

The hydrogel is a three-dimensional network structure formed bycrosslinking a hydrophilic polymer, and has a nature of being swollenwhen absorbing water. Here, a person skilled in the art may call thepolyvinyl alcohol-polyacrylic acid salt copolymer occasionally apolyvinyl alcohol-polyacrylic acid copolymer.

Even when the amount of water in the membrane is small, carbon dioxideis facilitatively transported, but its permeation rate is generally low,and therefore a large amount of water should be held in the membrane forachieving a high permeation rate. Further, when the gel membrane as aseparation-functional membrane is composed of a hydrogel having a highwater-holding capacity in the facilitated CO₂ transport membrane havingthe above-mentioned features, a maximum possible amount of water can beheld in the membrane even at a high temperature that causes a reductionin the amount of water in the separation-functional membrane, so thathigh selective permeability of CO₂ to hydrogen can be achieved at a hightemperature of 100° C. or higher.

Further, in the facilitated CO₂ transport membrane having theabove-mentioned features, the CO₂ carrier preferably contains at leastone of a carbonate of an alkali metal, a bicarbonate of an alkali metaland a hydroxide of an alkali metal, and further the alkali metal ispreferably cesium or rubidium. High selective permeability of CO₂ tohydrogen can be thereby achieved more reliably at a high temperature of100° C. or higher.

Here, a reaction expressed by the above (Chemical Formula 2) occurs whenthe CO₂ carrier is a carbonate of an alkali metal, while a reactionexpressed by the following (Chemical Formula 3) occurs when the CO₂carrier is a hydroxide of an alkali metal. The (Chemical Formula 3)shows a case where the alkali metal is cesium as an example.CO₂+CsOH→CsHCO₃CsHCO₃+CsOH→Cs₂CO₃+H₂O  (Chemical Formula 3)

The reactions in the above (Chemical Formula 3) can be united into areaction expressed by the (Chemical Formula 4). That is, this shows thatadded cesium hydroxide is converted into cesium carbonate. Further, itis apparent from the above (Chemical Formula 3) that a similar effectcan be obtained when as a CO₂ carrier, a bicarbonate is added in placeof a carbonate of an alkali metal.CO₂+2CsOH→Cs₂CO₃+H₂O  (Chemical Formula 4)

Further, in the facilitated CO₂ transport membrane having theabove-mentioned features, the separation-functional membrane ispreferably supported on a hydrophilic porous membrane.

First, when the separation-functional membrane is supported on a porousmembrane, the strength of the facilitated CO₂ transport membrane at thetime of use is improved. As a result, in the case where the facilitatedCO₂ transport membrane is applied to a CO₂ permeable membrane reactor(shift converter including a facilitated CO₂ transport membrane), asufficient membrane strength can be secured even when a pressuredifference between both sides (inside and outside of a reactor) of thefacilitated CO₂ transport membrane is large (e.g. 2 atm or larger).

Further, when the porous membrane supporting a separation-functionalmembrane as a gel membrane is hydrophilic, a gel membrane having reduceddefects can be stably prepared, so that high selective permeability tohydrogen can be maintained. In general, when the porous membrane ishydrophobic, it is supposed that penetration of water contained in thegel membrane into the pores of the porous membrane and the resultingreduction of membrane performance can be prevented at 100° C. or lower,and a similar effect may be expected at 100° C. or higher where theamount of water in the gel membrane is small. Therefore, use of ahydrophobic porous membrane is recommended. However, in the case of thefacilitated CO₂ transport membrane having the above-mentioned features,high selective permeability to hydrogen can be maintained with reduceddefects by using a hydrophilic porous membrane for the following reason.

When a cast solution including an aqueous solution containing ahydrophilic polymer such as a PVA/PAA salt copolymer and a CO₂ carrieris cast on a hydrophilic porous membrane, pores of the porous membraneare filled with the solution, and a surface of the porous membrane iscoated with the cast solution. When a separation-functional membrane isprepared by gelling the cast solution, not only a surface but also poresof the porous membrane are filled with the gel membrane, and thereforedefects are hard to occur, leading to an increase in gel membraneproduction success rate.

When considering the ratio of pore portions (porosity) and the situationin which the pore does not extend straight perpendicularly to themembrane surface but bends many times (bending rate), the gel membranein pores provides a great resistance to gas permeation, leading to areduction in gas permeance due to low permeability as compared to thegel membrane on the surface of the porous membrane. On the other hand,when a cast solution is cast on a hydrophobic porous membrane, pores ofthe porous membrane are not filled with the solution but only a surfaceof the porous membrane is coated with the cast solution, so that poresare filled with a gas, and therefore gas permeance in the gel layer onthe hydrophobic porous membrane is considered to be higher for both H₂and CO₂ as compared to a hydrophilic porous membrane.

However, minute defects easily occur in the gel membrane on the membranesurface as compared to the gel membrane in pores, leading to a reductionin membrane production success rate. H₂ is much smaller in molecularsize than CO₂, and therefore at a minute defect part, the permeance ofH₂ is remarkably larger than that of CO₂. At a part other than thedefect part, the permeance of CO₂ passing by the facilitated transportmechanism is considerably larger than the permeance of H₂ passing by thephysical solution-diffusion mechanism.

As a result, when a hydrophobic porous membrane is used, selectivity tohydrogen (CO₂/H₂) is reduced as compared to when a hydrophilic porousmembrane is used. Therefore, stability and durability of the facilitatedCO₂ transport membrane are very important from the viewpoint ofpractical use, and it is more advantageous to use a hydrophilic porousmembrane having high selectivity to hydrogen (CO₂/H₂).

Further, the separation-functional membrane supported on the hydrophilicporous membrane is preferably covered with a hydrophobic porousmembrane. The separation-functional membrane is thereby protected by thehydrophobic porous membrane, leading to a further increase in strengthof the facilitated CO₂ transport membrane at the time of use. Theseparation-functional membrane is covered with the hydrophobic porousmembrane, and therefore even when steam is condensed on the membranesurface of the hydrophobic porous membrane, water is repelled andthereby prevented from penetrating the separation-functional membranebecause the porous membrane is hydrophobic. Accordingly, the hydrophobicporous membrane can prevent a situation in which the CO₂ carrier in theseparation-functional membrane is diluted with water, and the dilutedCO₂ carrier flows out of the separation-functional membrane.

A cause which hinders downsizing and reduction of the startup time inconventional shift converters is that a large amount of a shift catalystis required due to the restriction from chemical equilibrium of the COshift reaction expressed by the following (Chemical Formula 5). As anexample, a reforming system for a 50 kW PAFC (phosphoric acid fuel cell)requires 20 L of a reforming catalyst, whereas the shift catalyst isrequired in an amount of 77 L, about 4 times the amount of the reformingcatalyst. This is a major factor of hindering downsizing and reductionof the startup time in the shift converter.CO+H₂O

CO₂+H₂  (Chemical Formula 5)

Thus, when the facilitated CO₂ transport membrane having theabove-mentioned features is applied to a CO₂ permeable membrane reactor,carbon dioxide on the right side, which is produced through the CO shiftreaction of the above (Chemical Formula 5), is efficiently removed tooutside the shift converter, so that chemical equilibrium can be shiftedto the hydrogen production side (right side) to obtain a high conversionrate at the same reaction temperature, and resultantly carbon monoxideand carbon dioxide can be removed beyond the limit imposed byequilibrium restriction. As a result, downsizing, reduction of thestartup time and velocity enhancement (SV enhancement) in the shiftconverter can be achieved.

Further, the present invention provides a method for producing thefacilitated CO₂ transport membrane having the above-mentioned features,the method comprising the steps of: preparing a cast solution includingan aqueous solution containing the hydrophilic polymer, the CO₂ carrierand the CO₂ hydration catalyst that is soluble in water; and casting thecast solution on a hydrophilic porous membrane and then gelling the castsolution to prepare the separation-functional membrane.

According to the method for producing the facilitated CO₂ transportmembrane having the above-mentioned features, since a cast solution isprepared beforehand in which the relative amounts of the CO₂ carrier andthe water-soluble CO₂ hydration catalyst to the hydrophilic polymer isproperly adjusted, proper adjustment of the blending ratio of the CO₂carrier and the CO₂ hydration catalyst in the final gel membrane can beeasily and conveniently achieved, so that performance of the membranecan be enhanced.

Further, the present invention provides a method for separating CO₂using the facilitated CO₂ transport membrane having the above-mentionedfeatures, with the CO₂ hydration catalyst having catalytic activity at atemperature of 100° C. or higher, wherein a mixed gas containing CO₂ andH₂ and having a temperature of 100° C. or higher is supplied to thefacilitated CO₂ transport membrane, and the CO₂ passing through thefacilitated CO₂ transport membrane is separated from the mixed gas.

Further, the present invention provides a CO₂ separation apparatuscomprising the facilitated CO₂ transport membrane having theabove-mentioned features, with the CO₂ hydration catalyst havingcatalytic activity at a temperature of 100° C. or higher, wherein amixed gas containing CO₂ and H₂ and having a temperature of 100° C. orhigher is supplied to the facilitated CO₂ transport membrane, and theCO₂ passing through the facilitated CO₂ transport membrane is separatedfrom the mixed gas.

EFFECTS OF THE INVENTION

According to the facilitated CO₂ transport membrane having theabove-mentioned features and the method for producing the same, afacilitated CO₂ transport membrane having an improved CO₂ permeance andan improved CO₂/H₂ selectivity can be stably supplied. Particularly, theCO₂ hydration catalyst has catalytic activity at a temperature of 100°C. or higher, so that a facilitated CO₂ transport membrane that isapplicable at a high temperature of 100° C. or higher and capable ofachieving high selective permeability to hydrogen can be stably suppliedin a decarbonation step in a hydrogen production process or the like.

Further, according to the CO₂ separation method and apparatus having theabove-mentioned features, a facilitated CO₂ transport membrane havinghigh selective permeability to hydrogen at a high temperature of 100° C.or higher is used, so that CO₂ can be selectively separated with highefficiency from a mixed gas containing CO₂ and H₂ and having atemperature of 100° C. or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically showing a structure in oneembodiment of a facilitated CO₂ transport membrane according to thepresent invention.

FIG. 2 is a flow chart showing a method for producing a facilitated CO₂transport membrane according to the present invention.

FIG. 3 is a table showing a list of constitutional conditions andmembrane performance for separation-functional membranes of Examples 1to 7 and Comparative Examples 1 and 2 used in experiments for evaluationof membrane performance of a facilitated CO₂ transport membraneaccording to the present invention.

FIG. 4 is a graph showing a CO₂ permeance and a CO₂/H₂ selectivity inExamples 1 to 5 and Comparative Example 1 shown in FIG. 3.

FIG. 5 is a graph showing a CO₂ permeance and a CO₂/H₂ selectivity inExamples 1 and 6 and Comparative Examples 1 and 2 shown in FIG. 3.

FIG. 6 is a graph showing a CO₂ permeance and a CO₂/H₂ selectivity inExamples 1 and 7 and Comparative Example 1 shown in FIG. 3.

FIG. 7 is a table showing a list of constitutional conditions andmembrane performance for separation-functional membranes of Examples 2,and 8 to 10 and Comparative Examples 1, 4 and 5 used in experiments forevaluation of membrane performance of a facilitated CO₂ transportmembrane according to the present invention.

FIG. 8 is a graph showing a CO₂ permeance and a CO₂/H₂ selectivity inExamples 2 and 8 and Comparative Examples 1 and 4 shown in FIG. 7.

FIGS. 9A and 9B are configuration diagrams each schematically showing anoutlined configuration in one embodiment of a CO₂ separation apparatusaccording to the present invention.

DESCRIPTION OF EMBODIMENTS

By extensively conducting studies, the inventors of the presentapplication have found that when a gel membrane of a facilitated CO₂transport membrane, which contains a CO₂ carrier and in which a reactionof CO₂ with the CO₂ carrier as expressed by the above (Chemical Formula2) occurs, contains a catalyst for a CO₂ hydration reaction as expressedby the above (Chemical Formula 1), one of elementary reactions of theabove-mentioned reaction, the catalyst being capable of maintainingcatalytic activity without being deactivated at a high temperature of100° C. or higher, the CO₂ permeance is dramatically improved withrespect to the H₂ permeance even at such a high temperature, and theCO₂/H₂ selectivity is considerably improved as compared to aconventional facilitated CO₂ transport membrane that does not containthe catalyst. Based on the above-mentioned new finding, the inventors ofthe present application have completed the invention of a facilitatedCO₂ transport membrane and a method for producing the same, and a methodand an apparatus for separating CO₂ as shown below.

[First Embodiment]

First, one embodiment of a facilitated CO₂ transport membrane and amethod for producing the same according to the present invention(hereinafter, referred to as “the present facilitated transportmembrane” and “the present production method” as appropriate) will bedescribed with reference to the drawings.

The present facilitated transport membrane is a facilitated CO₂transport membrane including a separation-functional membrane thatincludes a water-containing hydrophilic polymer gel membrane containinga CO carrier and a CO₂ hydration catalyst having catalytic activity at atemperature of 100° C. or higher, the facilitated CO₂ transport membraneserving at a temperature of 100° C. or higher and having a high COpermeance and a high CO₂/H₂ selectivity, and the facilitated CO₂transport membrane being applicable to a CO₂ permeable membrane reactoror the like. Further, for stably achieving a high CO₂/H₂ selectivity,the present facilitated transport membrane includes a hydrophilic porousmembrane as a support membrane that supports a gel membrane containing aCO₂ carrier and a CO₂ hydration catalyst.

Specifically, the present facilitated transport membrane includes apolyvinyl alcohol-polyacrylic acid (PVA/PAA) salt copolymer as amembrane material of the separation-functional membrane, a carbonate ofan alkali metal such as cesium carbonate (Cs₂CO₃) or rubidium carbonate(Rb₂CO₃) as the CO₂ carrier, and an oxo acid compound as the CO₂hydration catalyst. More specifically, for the CO₂ hydration catalyst,an oxo acid compound of at least one element selected from group 6elements, group 14 elements, group 15 elements and group 16 elements isused, and particularly preferably a tellurous acid compound, a seleniousacid compound, an arsenious acid compound, an orthosilicic acid compoundor a molybdic acid compound is used. All of CO₂ hydration catalysts usedin this embodiment are soluble in water, and extremely thermally stablewith a melting point of 400° C. or higher, and have catalytic activityat a high temperature of 100° C. or higher. The melting point of the CO₂hydration catalyst is only required to be higher than the upper limit oftemperature variations in steps in a method for producing the presentfacilitated transport membrane as described later (e.g. the temperaturein the drying step or thermal crosslinking temperature). When themelting point is, for example, about 200° C. or higher, a situation isavoided in which the CO₂ hydration catalyst is sublimed in the course ofthe production process, leading to a reduction in concentration of theCO₂ hydration catalyst in the separation-functional membrane.

As an example, the present facilitated transport membrane is configuredas a three-layer structure in which a hydrophilic porous membrane 2supporting a separation-functional membrane 1 is held between twohydrophobic porous membranes 3 and 4 as schematically shown in FIG. 1.The separation-functional membrane 1 as a gel membrane is supported onthe hydrophilic porous membrane 2 and has a certain level of mechanicalstrength, and therefore is not necessarily required to be held betweenthe two hydrophobic porous membranes 3 and 4. The mechanical strengthcan also be increased by, for example, forming the hydrophilic porousmembrane 2 in a cylindrical shape. Therefore, the present facilitatedtransport membrane is not necessarily a flat plate-shaped one.

The separation-functional membrane contains the PVA/PAA salt copolymerin an amount falling within a range of about 10 to 80% by weight, andthe CO₂ carrier in an amount falling within a range of about 20 to 90%by weight based on the total weight of the PVA/PAA salt copolymer andthe CO₂ carrier in the separation-functional membrane.

Further, the separation-functional membrane contains the CO₂ hydrationcatalyst, for example, in an amount of 0.01 times or more, preferably0.02 times or more, further preferably 0.025 times or more the amount ofthe CO₂ carrier in terms of molar number.

The hydrophilic porous membrane preferably has heat resistance to atemperature of 100° C. or higher, mechanical strength and adhesion withthe separation-functional membrane (gel membrane) in addition tohydrophilicity, and preferably has a porosity (void ratio) of 55% ormore and a pore size falling within a range of 0.1 to 1 μm. In thisembodiment, a hydrophilized tetrafluoroethylene polymer (PTFE) porousmembrane is used as a hydrophilic porous membrane that satisfies theabove-mentioned requirements.

The hydrophobic porous membrane preferably has heat resistance to atemperature of 100° C. or higher, mechanical strength and adhesion withthe separation-functional membrane (gel membrane) in addition tohydrophobicity, and preferably has a porosity (void ratio) of 55% ormore and a pore size falling within a range of 0.1 to 1 μm. In thisembodiment, a non-hydrophilized tetrafluoroethylene polymer (PTFE)porous membrane is used as a hydrophobic porous membrane that satisfiesthe above-mentioned requirements.

One embodiment of a method for producing the present facilitatedtransport membrane (the present production method) will now be describedwith reference to FIG. 2. The following descriptions are based on theassumption that a PVA/PAA salt copolymer is used as the hydrophilicpolymer, cesium carbonate (Cs₂CO₃) is used as the CO₂ carrier, and atellurite (e.g. potassium tellurite (K₂TeO₃)) is used as the CO₂hydration catalyst. The amounts of the hydrophilic polymer, the CO₂carrier and the CO₂ hydration catalyst are illustrative, and showamounts used in sample preparation in examples described below.

First, a cast solution including an aqueous solution containing aPVA/PAA salt copolymer, a CO₂ carrier and a CO₂ hydration catalyst isprepared (step 1). More specifically, 2 g of a PVA/PAA salt copolymer(e.g. provisional name: SS Gel manufactured by Sumitomo Seika ChemicalsCompany Limited), 4.67 g of cesium carbonate, and a tellurite in anamount of 0.025 times the amount of cesium carbonate in terms of molarnumber are added to 80 g of water, and the resultant mixture is stirreduntil they are dissolved, thereby obtaining a cast solution.

Next, the cast solution obtained in step 1 is cast on a hydrophilic PTFEporous membrane side surface of a layered porous membrane by anapplicator (step 2), the layered porous membrane being obtained byjoining two membranes: a hydrophilic PTFE porous membrane (e.g.WPW-020-80 manufactured by SUMITOMO ELECTRIC FINE POLYMER, INC.;thickness: 80 μm; pore size: 0.2 μm; void ratio: about 75%) and ahydrophobic PTFE porous membrane (e.g. FLUOROPORE FP010 manufactured bySUMITOMO ELECTRIC FINE POLYMER, INC.; thickness: 60 μm; pore size: 0.1μm; void ratio: 55%). The casting thickness in samples of examples andcomparative examples described later is 500 μm. Here, the cast solutionpenetrates pores in the hydrophilic PTFE porous membrane, but isinhibited from penetrating at the boundary surface of the hydrophobicPTFE porous membrane, so that the cast solution does not permeate to theopposite surface of the layered porous membrane, and there is no castsolution on a hydrophobic PTFE porous membrane side surface of thelayered porous membrane. This makes handling easy.

Next, the hydrophilic PTFE porous membrane after casting is naturallydried at room temperature, and the cast solution is then gelled toproduce a separation-functional membrane (step 3). Here, gelation meansthat the cast solution as a polymer dispersion liquid is dried into asolid form, and the gel membrane is a solid membrane produced by thegelation, and is clearly distinguished from a liquid membrane.

In the present production method, the cast solution is cast on ahydrophilic PTFE porous membrane side surface of the layered porousmembrane in step 2, and therefore the separation-functional membrane isnot only formed on a surface (cast surface) of the hydrophilic PTFEporous membrane but also formed so as to fill pores in step 3, so thatdefects (minute defects such as pinholes) are hard to occur, leading toan increase in membrane production success rate of theseparation-functional membrane. It is desirable to further thermallycrosslink the naturally dried PTFE porous membrane at about 120° C. forabout 2 hours in step 3. All of samples in examples and comparativeexamples described later are thermally crosslinked.

Next, a hydrophobic PTFE porous membrane identical to the hydrophobicPTFE porous membrane of the layered porous membrane used in step 2 issuperimposed on a gel layer side surface of the hydrophilic PTFE porousmembrane obtained in step 3 to obtain the present facilitated transportmembrane of three layer structure including a hydrophobic PTFE porousmembrane/a separation-functional membrane supported on a hydrophilicPTFE porous membrane/a hydrophobic PTFE porous membrane as schematicallyshown in FIG. 1 (step 4). FIG. 1 schematically and linearly shows astate in which the separation-functional membrane 1 fills pores of thehydrophilic PTFE porous membrane 2.

In the present production method, the blending ratio of the CO₂ carrierand the CO₂ hydration catalyst can be adjusted in step 1 of producing acast solution, and therefore, as compared to a case where afterformation of a gel membrane that does not contain at least one of theCO₂ carrier and the CO₂ hydration catalyst, at least one of the CO₂carrier and the CO₂ hydration catalyst is added into the gel membrane,adjustment of the blending ratio can be more accurately and easilyperformed, leading to enhancement of membrane performance.

Thus, the present facilitated transport membrane prepared by followingsteps 1 to 4 can exhibit extremely high selective permeability tohydrogen even at a high temperature of 100° C. or higher, for example aCO₂ permeance of about 3×10⁻⁵ mol/(m²·s·kPa) (=90 GPU) or more and aCO₂/H₂ selectivity of about 100 or more.

Hereinafter, specific membrane performance of the present facilitatedtransport membrane is evaluated by comparing Examples 1 to 7 in whichthe separation-functional membrane contains a CO₂ hydration catalystwith Comparative Examples 1 and 2 in which the separation-functionalmembrane does not contain a CO₂ hydration catalyst.

The samples in Examples 1 to 7 and Comparative Examples 1 and 2 belowwere prepared in accordance with the present production method describedabove. The weights of the solvent (water), the hydrophilic polymer andthe CO₂ carrier in the cast solution prepared in step 1 are the sameamong Examples 1 to 7 and Comparative Examples 1 and 2. As thehydrophilic polymer, a PVA/PAA salt copolymer was used. As the CO₂carrier, cesium carbonate (Cs₂CO₃) is used except for Example 6, and theweight ratio of cesium carbonate to the total weight of the PVA/PAA saltcopolymer and cesium carbonate (carrier concentration) is 70% by weightin each of the examples and comparative examples. In Example 6, rubidiumcarbonate (Rb₂CO₃) is used as the CO₂ carrier, and the weight ratio ofrubidium carbonate to the total weight of the PVA/PAA salt copolymer (2g) identical to that in Example 1 and rubidium carbonate (4.67 g)(carrier concentration) is 70% by weight.

In Examples 1, 6 and 7, potassium tellurite (melting point: 465° C.) wasused as the CO₂ hydration catalyst. In Examples 2 to 5, lithiumtellurite (Li₂O₃Te, melting point: about 750° C.), potassium selenite(K₂O₃Se, melting point: 875° C.), sodium arsenite (NaO₂As, meltingpoint: 615° C.) and sodium orthosilicate (Na₄O₄Si, melting point: 1018°C.) were used, respectively, as the CO₂ hydration catalyst. The molarratio of the CO₂ hydration catalyst to the CO₂ carrier is 0.025 inExamples 1 to 5, 0.05 in Example 6, and 0.2 in Example 7.

The sample in Comparative Example 1 was prepared in the same manner asin Example 1 except that the cast solution prepared in step 1 in theproduction method described above did not contain a CO₂ hydrationcatalyst. The sample in Comparative Example 2 was prepared in the samemanner as in Example 6 except that the cast solution prepared in step 1in the production method described above did not contain a CO₂ hydrationcatalyst.

An experiment method for evaluating membrane performance of the samplesin Examples 1 to 7 and Comparative Examples 1 and 2 will now bedescribed.

Each sample was used while being fixed between a supply side chamber anda permeate side chamber in a stainless steel flow type gas permeationcell using a fluororubber gasket as a seal material. Experimentalconditions are the same for the samples, and the temperature of theinside of the cell is fixed at 130° C.

The supply side gas supplied to the supply side chamber is a mixed gasincluding CO₂, H₂ and H₂O (steam), and the ratio (mol %) among them isCO₂:H₂:H₂O=23.6:35.4:41.0. The flow rate of the supply side gas is3.47×10⁻² mol/min. and the supply side pressure is 600 kPa (A). (A)means an absolute pressure. Accordingly, the CO₂ partial pressure on thesupply side is 142 kPa (A). The pressure of the supply side chamber isadjusted with a back pressure regulator provided on the downstream sideof a cooling trap at some midpoint in an exhaust gas dischargingpassage.

On the other hand, the pressure of the permeate side chamber isatmospheric pressure. H₂O (steam) is used as a sweep gas made to flowinto the permeate side chamber, and the flow rate thereof is 7.77×10⁻³mol/min. For sending the sweep gas discharged from the permeate sidechamber to a gas chromatograph on the downstream side, an Ar gas isinpoured, steam in the gas containing the Ar gas is removed by thecooling trap, the composition of the gas after passing through thecooling trap is quantitatively determined by the gas chromatograph, thepermeance [mol/(m²·s·kPa)] of each of CO₂ and H₂ is calculated from thecomposition and the flow rate of Ar in the gas, and from the ratiothereof, the CO₂/H₂ selectivity is calculated.

In the evaluation experiment described above, the experiment apparatushas a pre-heater for heating the gas and the flow type gas permeationcell with a sample membrane fixed therein is placed in a thermostaticoven in order to keep constant the use temperature of the presentfacilitated transport membrane of each sample and the temperatures ofthe supply side gas and the sweep gas.

Next, comparison of membrane performance obtained in experiment resultsin Examples 1 to 7 and Comparative Examples 1 and 2 is made. FIG. 3shows a list of constitutional conditions (CO₂ carrier, CO₂ hydrationcatalyst, molar ratio of CO₂ carrier to CO₂ hydration catalyst,hydrophilic polymer) and membrane performance (CO₂ permeance. H₂permeance and CO₂/H₂ selectivity) for separation-functional membranes ofthe samples in Examples 1 to 7 and Comparative Examples 1 and 2.

First, comparison of membrane performance is made among Examples 1 to 5and Comparative Example 1. Here, comparison of membrane performanceassociated with presence/absence of the CO₂ hydration catalyst and thetype thereof is made. FIG. 4 shows, in the form of a graph, the CO₂permeance and CO₂/H₂ selectivity in Examples 1 to 5 and ComparativeExample 1. It is apparent from FIGS. 3 and 4 that since theseparation-functional membrane contains a CO₂ hydration catalyst, theCO₂ permeance increases by a factor of 1.14 to 1.76, while the H₂permeance increases by a factor of 0.72 to 1.29, and the increasing rateof CO₂ permeance is greater than that of H₂ permeance, so that theCO₂/H₂ selectivity is improved to fall within a range of 104 to 135 ascompared to a CO₂/H₂ selectivity of 79.2 in Comparative Example 1.

While from FIG. 4, all of the CO₂ hydration catalysts are confirmed toimprove both the CO₂ permeance and CO₂/H₂ selectivity, the CO₂ permeanceis remarkably improved when a tellurite is used.

Since the CO₂ hydration catalyst is a catalyst for increasing thereaction rate of a CO₂ hydration reaction expressed by the above(Chemical Formula 1), it is considered that when theseparation-functional membrane contains a CO₂ hydration catalyst, areaction of CO₂ with a CO₂ carrier, which includes the CO₂ hydrationreaction as one of elementary reactions and which is expressed by theabove (Chemical Formula 2), is accelerated, leading to an increase inCO₂ permeance by the facilitated transport mechanism. This is consistentwith the experiment results shown in FIG. 3. However, since H₂ does notreact with the CO₂ carrier as described above, the H₂ permeationmechanism may be based on the solution-diffusion mechanism rather thanthe facilitated transport mechanism, and it is considered that the H₂permeance is not directly affected by presence/absence of the CO₂hydration catalyst, the blending ratio and type thereof, and the like.Further, the samples in Examples 1 to 7 and Comparative Examples 1 and 2are different in constitutional conditions for the separation-functionalmembrane, and are therefore each individually prepared. Therefore,differences in measurement value of H₂ permeance among the samples areconsidered to mainly result from individual differences (variations) inmembrane quality of the hydrophilic polymer gel membrane. It is to benoted that the H₂ permeance may be indirectly affected by influences onmembrane quality of the hydrophilic polymer gel membrane given bydifferences in amount, type and the like of the CO₂ carrier and the CO₂hydration catalyst in addition to the individual differences in membranequality.

Next, comparison of membrane performance is made among Examples 1 and 6and Comparative Examples 1 and 2. Here, comparison of membraneperformance associated with presence/absence of the CO₂ hydrationcatalyst and the type the CO₂ carrier is made. FIG. 5 shows, in the formof a graph, the CO₂ permeance and CO₂/H₂ selectivity in Examples 1 and 6and Comparative Examples 1 and 2. FIG. 5 shows that when theseparation-functional membrane does not contain a CO₂ hydrationcatalyst, there is no significant difference in performance due to adifference in CO₂ carrier with the separation-functional membrane havinga CO₂ permeance of 2.83 to 2.84×10⁻⁵ (mol/(m²·s·kPa)), a H₂ permeance of3.05 to 3.58×10⁻⁷ (mol/(m²·s·kPa)) and a CO₂/H₂ selectivity of 79.2 to93.1 in both cases where the CO₂ carrier is cesium carbonate and wherethe CO₂ carrier is rubidium carbonate. When the CO₂ carrier is cesiumcarbonate, performance is greatly improved with the CO₂ permeanceincreasing by a factor of 1.53, the H₂ permeance increasing by a factorof 1.02 and the CO₂/H₂ selectivity increasing by a factor of 1.50because the separation-functional membrane contains a CO₂ hydrationcatalyst. When the CO₂ carrier is rubidium carbonate, performance isgreatly improved as in the case where the CO₂ carrier is cesiumcarbonate, with the CO₂ permeance increasing by a factor of 1.68, the H₂permeance increasing by a factor of 0.83 and the CO₂/H₂ selectivityincreasing by a factor of 2.04. The reason why in Example 6, the H₂permeance decreases to 0.83 times that in Comparative Example 2 may bebecause of individual differences in membrane quality of the hydrophilicpolymer gel membrane.

Next, comparison of membrane performance is made among Examples 1 and 7and Comparative Example 1. Here, comparison of membrane performanceassociated with presence/absence of the CO₂ hydration catalyst, and theblending ratio thereof (molar ratio to cesium carbonate) is made. FIG. 6shows, in the form of a graph, the CO₂ permeance and CO₂/H, selectivityin Examples 1 and 7 and Comparative Example 1.

When comparison is made among Comparative Example 1 and Examples 1 and7, it is apparent that both the CO₂ permeance and CO₂/H₂ selectivity areimproved as the blending ratio of the CO₂ hydration catalyst (potassiumtellurite) increases.

As a result of measuring the CO₂ permeance with another sample in whichthe molar ratio of the CO₂ hydration catalyst to the CO₂ carrier isdecreased to 0.01 when the hydrophilic polymer is a PVA/PAA saltcopolymer, the CO₂ carrier is cesium carbonate and the CO₂ hydrationcatalyst is potassium tellurite, aside from Examples 1 and 7, it hasbeen confirmed that the CO₂ permeance was improved to 3.74×10⁻⁵(mol/(m²·s·kPa)). i.e. 1.32 times the CO₂ permeance in ComparativeExample 1.

While all of the separation-functional membranes in Examples 1 to 7 andComparative Examples 1 and 2 are gel membranes, Comparative Example 3having a liquid membrane (aqueous solution) as a separation-functionalmembrane was prepared as another comparative example. The aqueoussolution of a separation-functional membrane in Comparative Example 3does not contain the PVA/PAA salt copolymer used in Examples 1 to 7 andComparative Example 1. In Comparative Example 3, cesium carbonate wasused as a CO₂ carrier and potassium tellurite was used as a CO₂hydration catalyst similarly to Example 1. Hereinafter, a method forpreparing Comparative Example 3 will be described.

To an aqueous cesium carbonate solution having a molar concentration of2 mol/L was added potassium tellurite in an amount of 0.025 times theamount of cesium carbonate in terms of molar number, and the resultantmixture was stirred until potassium tellurite was dissolved, therebyobtaining an aqueous solution for a separation-functional membrane(liquid membrane). Thereafter, instead of the casting method using anapplicator in step 2 in the present production method, a hydrophilicPTFE porous membrane was immersed in the aqueous solution for aseparation-functional membrane (liquid membrane) for 30 minutes, and thehydrophilic PTFE membrane soaked with the aqueous solution was thenplaced on a hydrophobic PTFE membrane, and dried at room temperature forhalf a day or longer. Similarly to Examples 1 to 7 and ComparativeExamples 1 and 2, another hydrophobic PTFE membrane is placed on thehydrophilic PTFE membrane to form a three-layer structure with thehydrophilic PTFE porous membrane and the separation-functional membrane(liquid membrane) held between the hydrophobic PTFE membranes at thetime of an experiment for evaluation of membrane performance.

However, in the case of the liquid membrane sample of ComparativeExample 3, it was impossible to set the supply side pressure of 600 kPa(A), i.e. an experimental condition similar to that in Examples 1 to 7and Comparative Examples 1 and 2, and membrane performance could not beevaluated. That is, it became evident that a necessary differentialpressure cannot be maintained because the difference in pressure betweenthe supply side and the permeate side in the separation-functionalmembrane (liquid membrane) cannot be endured.

Thus, by comparing membrane performance between Examples 1 to 7 in whichthe separation-functional membrane contains a CO₂ hydration catalyst andComparative Examples 1 and 2 in which the separation-functional membranedoes not contain a CO₂ hydration catalyst, an effect of considerablyimproving the CO₂ permeance and CO₂/H₂ selectivity was confirmed as thepresent facilitated transport membrane includes a CO₂ hydration catalystin the separation-functional membrane. Here, the facilitated CO₂transport membrane has such characteristics that in a certain thicknessrange, thickness dependency is kept low, so that the permeation rate ofCO₂ hardly decreases even when the thickness increases. On the otherhand, H₂ passes through the separation-functional membrane by thesolution-diffusion mechanism as described above, and therefore itspermeation rate tends to be inversely proportional to the membranethickness. Therefore, further improvement of the CO₂/H₂ selectivity isexpected due to the synergistic effect of the advantage that the effectof improving the CO₂ permeance due to presence of a CO₂ hydrationcatalyst in the separation-functional membrane is attained withoutdepending on the membrane thickness and the advantage that the H₂permeance is reduced as the thickness is increased.

Results of evaluating membrane performance in Examples 8 and 9 in whichthe separation-functional membrane prepared with a thickness that isabout 2 times the thickness in Examples 1 to 7 and Comparative Examples1 and 2 contains a CO₂ hydration catalyst and Comparative Example 4 inwhich the separation-functional membrane does not contain a CO₂hydration catalyst will now be described.

The samples in Examples 8 and 9 and Comparative Example 4 were preparedin accordance with the present production method described above. It isto be noted that a series of steps including step 2 and step 3 wererepeated twice for increasing the thickness of the separation-functionalmembrane. The weights of the solvent (water), the hydrophilic polymerand the CO₂ carrier in the cast solution prepared in step 1 are the sameamong Examples 8 and 9 and Comparative Example 4, and identical to thosein Examples 1 to 7 and Comparative Examples 1 and 2. In each of Examples8 and 9 and Comparative Example 4, cesium carbonate (Cs₂CO₃) is used asthe CO₂ carrier, and the weight ratio of cesium carbonate to the totalweight of the PVA/PAA salt copolymer and cesium carbonate (carrierconcentration) is 70% by weight.

In Examples 8 and 9, lithium tellurite and potassium molybdate (K₂O₄Mo,melting point: about 919° C.) were used in this order as the CO₂hydration catalyst. The molar ratio of the CO₂ hydration catalyst to theCO₂ carrier is 0.025 in Example 8, and 0.1 in Example 9. The sample inComparative Example 4 was prepared in the same manner as in Example 8except that the cast solution prepared in step 1 in the productionmethod described above did not contain a CO₂ hydration catalyst.

An experiment method for evaluating membrane performance of the samplesin Examples 8 and 9 and Comparative Example 4 is identical to theexperiment method for evaluating membrane performance of the samples inExamples 1 to 7 and Comparative Examples 1 and 2 described aboveincluding the gas composition and ratio of the supply side gas, the gasflow rate, the pressure, the use temperature and so on.

FIG. 7 shows a list of constitutional conditions (CO₂ carrier, CO₂hydration catalyst, molar ratio of CO₂ carrier to CO₂ hydrationcatalyst, hydrophilic polymer) and membrane performance (CO₂ permeance,H₂ permeance and CO₂/H₂ selectivity) for separation-functional membranesof the samples in Examples 2, 8 and 9 and Comparative Examples 1 and 4.FIG. 8 shows, in the form of a graph, the CO₂ permeance and CO₂/H₂selectivity in Examples 2 and 8 and Comparative Examples 1 and 4.

First, when comparison of membrane performance is made betweenComparative Example 4 and Comparative Example 1, the membrane thicknessin Comparative Example 4 is about 2 times the membrane thickness inComparative Example 1, but there is no difference in otherconstitutional conditions of the separation-functional membrane, andtherefore there is substantially no difference in CO₂ permeance as it isnot significantly influenced by the membrane thickness, whereas the H₂permeance is much lower in Comparative Example 4 than in ComparativeExample 1 due to the about 2-fold difference in membrane thickness. As aresult, the CO₂/H₂ selectivity is higher in Comparative Example 4 thanin Comparative Example 1. Similarly, when comparison of membraneperformance is made between Example 8 and Example 2, the membranethickness in Example 8 is about 2 times the membrane thickness inExample 2, but there is no difference in other constitutional conditionsof the separation-functional membrane, and therefore there issubstantially no difference in CO₂ permeance as it is not significantlyinfluenced by the membrane thickness, and an effect of improving the CO₂permeance by the CO₂ hydration catalyst is similarly attained, whereasthe H₂ permeance is much lower in Example 8 than in Example 2 due to theabout 2-fold difference in membrane thickness. As a result, the CO₂/H₂selectivity is higher in Example 8 than in Example 2. When comparison ofmembrane performance is made between Example 8 and Comparative Example4, it is apparent that similarly to considerable improvement of the CO₂permeance and CO₂/H₂ selectivity in Example 2 as compared to ComparativeExample 1, the CO₂ permeance and CO₂/H₂ selectivity are considerablyimproved even when the thickness of the separation-functional membraneis large. That is, it has become evident that the effect of improvingthe CO₂ permeance due to presence of a CO₂ hydration catalyst in theseparation-functional membrane is attained without depending on thethickness of the separation-functional membrane in a certain thicknessrange.

Next, when comparison is made between Example 9 and Comparative Example4, an effect of improving the CO₂ permeance and CO₂/H₂ selectivity dueto presence of a CO₂ hydration catalyst in the separation-functionalmembrane can be confirmed even with a membrane thickness that is about 2times the membrane thickness in Examples 1 to 7 also when the CO₂hydration catalyst is potassium molybdate.

Here, the CO₂ hydration catalyst in each of Examples 1 to 3 and 6 to 8and Example 10 described later is an oxo acid compound of a group 16element, the CO₂ hydration catalyst in Example 4 is an oxo acid compoundof a group 15 element, the CO₂ hydration catalyst in Example 5 is an oxoacid compound of a group 14 element, and the CO₂ hydration catalyst inExample 9 is an oxo acid compound of a group 6 element. Accordingly,from the results of evaluating membrane performance, it is apparent thatat least oxo acid compounds of group 6 elements, group 14 elements,group 15 elements and group 16 elements suitably include a CO₂ hydrationcatalyst which is soluble in water and extremely thermally stable with amelting point of 200° C. or higher, and has catalytic activity at a hightemperature of 100° C. or higher. However, this does not mean that allthe oxo acid compounds of group 6 elements, group 14 elements, group 15elements and group 16 elements have catalytic activity as a CO₂hydration catalyst, and the possibility is not ruled out that oxo acidcompounds other than those of group 6 elements, group 14 elements, group15 elements and group 16 elements include those which have catalyticactivity as a CO₂ hydration catalyst and can be used for the presentfacilitated transport membrane.

Further, as substances having catalytic activity as a CO₂ hydrationcatalyst, there are many substances other than oxo acid compounds, suchas enzymes. Therefore, the CO₂ hydration catalyst is not limited to theoxo acid compounds used in Examples 1 to 9 as long as it can be suitablyused for the present facilitated transport membrane. Here, as an exampleof conditions suitable for the present facilitated transport membrane asa CO₂ hydration catalyst, the substance is soluble in water, andextremely thermally stable with a melting point of 200° C. or higher,and has catalytic activity at a high temperature of 100° C. or higher.

In the above-mentioned embodiment, as an example of a suitableconfiguration of the present facilitated transport membrane, aconfiguration has been shown in which a hydrogel of a PVA/PAA saltcopolymer as a hydrophilic polymer is used as a membrane material of aseparation-functional membrane, and a hydrophilic porous membrane isused as a porous membrane that supports the separation-functionalmembrane. However, since the hydrophilic polymer gel membrane contains aCO₂ hydration catalyst, the effect of improving the CO₂ permeance andCO₂/H₂ selectivity can also be exhibited, although varying in level,when a hydrophilic polymer other than PVA/PAA salt copolymers, such as,for example, polyvinyl alcohol (PVA) or a polyacrylic acid (PAA) salt isused, or when a hydrophobic porous membrane is used as a porous membranethat supports the separation-functional membrane.

Results of evaluating membrane performance in Example 10 in which theseparation-functional membrane contains a CO₂ hydration catalyst andComparative Example 5 in which the separation-functional membrane doesnot contain a CO₂ hydration catalyst, with polyvinyl alcohol (PVA) beingused as a hydrophilic polymer in both Example 10 and Comparative Example5, will now be described.

The samples in Example 10 and Comparative Example 5 were prepared inaccordance with the present production method described above. It is tobe noted that similarly to Examples 8 and 9 and Comparative Example 4, aseries of steps including step 2 and step 3 were repeated twice forincreasing the thickness of the separation-functional membrane. Theweights of the solvent (water), the hydrophilic polymer and the CO₂carrier in the cast solution prepared in step 1 are the same betweenExample 10 and Comparative Example 5. In each of Example 10 andComparative Example 5, cesium carbonate (Cs₂CO₃) is used as the CO₂carrier, and the weight ratio of cesium carbonate to the total weight ofPVA and cesium carbonate (carrier concentration) is 46% by weight. Thepolymerization degree of polyvinyl alcohol used is about 2000, and theporous membrane supporting the separation-functional membrane is a PTFEporous membrane having a pore size of 0.1 μm and a thickness of 50 μm.

In Example 10, potassium tellurite is used as a CO₂ hydration catalyst,and the molar ratio of the CO₂ hydration catalyst to the CO₂ carrier is0.2. The sample in Comparative Example 5 was prepared in the same manneras in Example 10 except that the cast solution prepared in step 1 in theproduction method described above did not contain a CO₂ hydrationcatalyst.

An experiment method for evaluating membrane performance of the samplesin Example 10 and Comparative Example 5 is identical to the experimentmethod for evaluating membrane performance of the samples in Examples 1to 9 and Comparative Examples 1, 2 and 4 described above except for theratio of gas components of the supply side gas, the supply side gas flowrate, the supply side pressure and the use temperature. The ratio (mol%) among CO₂, H₂ and H₂O (steam) in the supply side gas supplied to thesupply side chamber is CO₂:H₂:H₂O=5.0:48.7:46.3. The flow rate of thesupply side gas is 6.14×10⁻² mol/min, the supply side pressure is 300kPa (A), and the temperature of the inside of the flow type gaspermeation cell is fixed at 120° C.

FIG. 7 shows a list of constitutional conditions (CO₂ carrier, CO₂hydration catalyst, molar ratio of CO₂ carrier to CO₂ hydrationcatalyst, hydrophilic polymer) and membrane performance (CO₂ permeance,H₂ permeance and CO₂/H₂ selectivity) for separation-functional membranesof the samples in Example 10 and Comparative Example 5.

When comparison of membrane performance is made between Example 10 andComparative Example 5, it is apparent that the CO₂ permeance and CO₂/H₂selectivity are considerably improved. From this result, it has becomeevident that the effect of improving the CO₂ permeance due to presenceof a CO₂ hydration catalyst in the separation-functional membrane isattained also when polyvinyl alcohol is used as the hydrophilic polymer.Accordingly, it is well conceivable that the effect of improving the CO₂permeance is attained irrespective of the composition of the hydrophilicpolymer. Therefore, the hydrophilic polymer that forms theseparation-functional membrane of the present facilitated transportmembrane is not limited to the PVA/PAA salt copolymer and polyvinylalcohol (PVA) shown as examples in the above-mentioned embodiment.

[Second Embodiment]

A CO₂ separation apparatus and a CO₂ separation method, to which thefacilitated CO₂ transport membrane described in the first embodiment isapplied, will now be described with reference to FIGS. 9A and 9B.

FIGS. 9A and 9B are each a sectional view schematically showing anoutlined structure of a CO₂ separation apparatus 10 of this embodiment.In this embodiment, as an example, a facilitated CO₂ transport membranemodified into a cylindrical structure is used instead of the facilitatedCO₂ transport membrane of flat plate structure described in the firstembodiment. FIG. 9A shows a cross section structure at a cross sectionperpendicular to the axial center of a facilitated CO₂ transportmembrane (the present facilitated transport membrane) 11 of cylindricalstructure, and FIG. 9B shows a cross section structure at a crosssection extending through the axial center of the present facilitatedtransport membrane 11.

The present facilitated transport membrane 11 shown in FIGS. 9A and 9Bhas a structure in which a separation-functional membrane 1 is supportedon the outer circumferential surface of a cylindrical hydrophilicceramic porous membrane 2. Similarly to the first embodiment, theseparation-functional membrane 1 includes a polyvinylalcohol-polyacrylic acid (PVA/PAA) salt copolymer as a membrane materialof the separation-functional membrane, a carbonate of an alkali metalsuch as cesium carbonate (Cs₂CO₃) or rubidium carbonate (Rb₂CO₃) as theCO₂ carrier, and a tellurous acid compound, a selenious acid compound,an arsenious acid compound and an orthosilicic acid compound as the CO₂hydration catalyst. The membrane structure in this embodiment isdifferent from the membrane structure in the first embodiment in thatthe separation-functional membrane 1 and the hydrophilic ceramic porousmembrane 2 are not held between two hydrophobic porous membranes. Themethod for producing the separation-functional membrane 1 and membraneperformance thereof in this embodiment are basically similar to those inthe first embodiment except for the above-mentioned difference, andtherefore duplicate explanations are omitted.

As shown in FIGS. 9A and 9B, the present cylindrical facilitatedtransport membrane 11 is housed in a bottomed cylindrical container 12,and a supply side space 13 surrounded by the inner wall of the container12 and the separation-functional membrane 1 and a permeate side space 14surrounded by the inner wall of the ceramic porous membrane 2 areformed. A first feeding port 15 for feeding a source gas FG into thesupply side space 13 and a second feeding port 16 for feeding a sweepgas SG into the permeate side space 14 are provided on one of bottomportions 12 a and 12 b on opposite sides of the container 12, and afirst discharge port 17 for discharging a CO₂-separated source gas EGfrom the supply side space 13 and a second discharge port 18 fordischarging from the permeate side space 14 a discharge gas SG′including a mixture of a CO₂-containing permeate gas PG passing throughthe present facilitated transport membrane 11 and the sweep gas SG areprovided on the other of the bottom portions 12 a and 12 b on oppositesides of the container 12. The container 12 is made of, for example,stainless steel, and although not illustrated, the present facilitatedtransport membrane 11 is fixed in the container 12 with a fluororubbergasket interposed as a seal material between opposite ends of thepresent facilitated transport membrane 11 and the inner walls of thebottom portions 12 a and 12 b on opposite sides of the container 12similarly to the experiment apparatus described in the first embodimentas an example. The method for fixing the present facilitated transportmembrane 11 and the sealing method are not limited to the methodsdescribed above.

In FIG. 9B, each of the first feeding port 15 and the first dischargeport 17 is provided in each of the supply side spaces 13 illustratedseparately on the left and the right in FIG. 9B. However, since thesupply side spaces 13 annularly communicate with each other as shown inFIG. 9A, the first feeding port 15 and the first discharge port 17 maybe provided in one of the left and right supply side spaces 13. Further,FIG. 9B shows as an example a configuration in which the first feedingport 15 and the second feeding port 16 are provided on one of the bottomportions 12 a and 12 b, and the first discharge port 17 and the seconddischarge port 18 are provided on the other of the bottom portions 12 aand 12 b, but a configuration may be employed in which the first feedingport 15 and the second discharge port 18 are provided on one of thebottom portions 12 a and 12 b, and the first discharge port 17 and thesecond feeding port 16 are provided on the other of the bottom portions12 a and 12 b. That is, the direction along which the source gases FGand EG flow and the direction along which the sweep gas SG and thedischarge gas SG′ flow may be reversed.

In the CO₂ separation method of this embodiment, the source gas FGincluding a mixed gas containing CO₂ and H₂ and having a temperature of100° C. or higher is fed into the supply side space 13 and therebysupplied to the supply side surface of the present facilitated transportmembrane 11, so that a CO₂ carrier contained in theseparation-functional membrane 1 of the present facilitated transportmembrane 11 is reacted with CO₂ in the source gas FG to allow selectivepassage of CO, at a high selection ratio to hydrogen, and theCO₂-separated source gas EG having an increased H₂ concentration isdischarged from the supply side space 13.

The reaction of CO₂ with the CO₂ carrier requires supply of water (H₂O)as shown in the above reaction formula of (Chemical Formula 2), and asthe amount of water contained in the separation-functional membrane 1increases, chemical equilibrium is shifted to the product side (rightside), so that permeation of CO₂ is facilitated. When the temperature ofthe source gas FG is a high temperature of 100° C. or higher, theseparation-functional membrane 1 that is in contact with the source gasFG is also exposed to a high temperature of 100° C. or higher, so thatwater contained in the separation-functional membrane 1 is evaporatedand passes into the permeate side space 14 similarly to CO₂, andtherefore it is necessary to supply steam (H₂O) to the supply side space13. The steam may be contained in the source gas FG, or may be suppliedto the supply side space 13 independently of the source gas FG. In thelatter case, steam (H₂O) passing into the permeate side space 14 may beseparated from the discharge gas SG′ and circulated into the supply sidespace 13.

For the CO₂ separation apparatus shown in FIGS. 9A and 9B, aconfiguration example has been described in which the supply side space13 is formed at the outside while the permeate side space 14 is formedat the inside of the present cylindrical facilitated transport membrane11, but the supply side space 13 may be formed at the inside while thepermeate side space 14 may be formed at the outside. The presentfacilitated transport membrane 11 may have a structure in which theseparation-functional membrane 1 is supported on the innercircumferential surface of the cylindrical hydrophilic ceramic porousmembrane 2. Further, the present facilitated transport membrane 11 usedin the CO₂ separation apparatus is not necessarily cylindrical, but maybe in the form of a tube having a cross-sectional shape other than acircular shape, and the present facilitated transport membrane of flatplate structure as shown in FIG. 1 may be used.

As an application example of the CO₂ separation apparatus described inthis embodiment, a shift converter (CO₂ permeable membrane reactor)including the present facilitated transport membrane will now be brieflydescribed.

For example, when a CO₂ permeable membrane reactor is formed using theCO₂ separation apparatus 10 shown in FIGS. 9A and 9B, the supply sidespace 13 can be used as a shift converter by filling the supply sidespace 13 with a shift catalyst.

The CO₂ permeable membrane reactor is an apparatus in which, forexample, a source gas FG produced in a steam reforming device and havingH₂ as a main component is received in the supply side space 13 filledwith a shift catalyst, and carbon monoxide (CO) contained in the sourcegas FG is removed through a CO shift reaction expressed by the above(Chemical Formula 5). CO₂ produced through the CO shift reaction isallowed to permeate to the permeate side space 14 selectively by meansof the present facilitated transport membrane 11 and removed, wherebychemical equilibrium can be shifted to the hydrogen production side, sothat CO and CO₂ can be removed beyond the limit imposed by equilibriumrestriction with a high conversion rate at the same reactiontemperature. A source gas EG freed of CO and CO₂ and having H₂ as a maincomponent is taken out from the supply side space 13.

Since the performance of the shift catalyst used for the CO shiftreaction tends to decrease with a decrease in temperature, the usetemperature is considered to be 100° C. at minimum, and the temperatureof the source gas FG supplied to the supply side surface of the presentfacilitated transport membrane 11 is 100° C. or higher. Therefore, thesource gas FG is adjusted to a temperature suitable for catalyticactivity of the shift catalyst, then fed into the supply side space 13filled with the shift catalyst, subjected to the CO shift reaction(exothermic reaction) in the supply side space 13, and supplied to thepresent facilitated transport membrane 11.

On the other hand, the sweep gas SG is used for maintaining the drivingforce for the permeation through the present facilitated transportmembrane 11 by lowering the partial pressure of the CO₂-containingpermeate gas PG which permeates the present facilitated transportmembrane 11 and for discharging the permeate gas PG to the outside. Itis to be noted that when the partial pressure of the source gas FG issufficiently high, it is not necessary to feed the sweep gas SG becausea partial pressure difference serving as the driving force forpermeation is obtained even if the sweep gas SG is not fed. As a gasspecies used for the sweep gas, steam (H₂O) can also be used as in thecase of the experiment for evaluation of membrane performance in thefirst embodiment, and further an inert gas such as Ar can also be used.The sweep gas SG is not limited to a specific gas species.

[Other Embodiments]

Hereinafter, other embodiments will be described.

<1> The above-mentioned embodiments have been described based on theassumption that a carbonate, a bicarbonate or a hydroxide of an alkalimetal such as cesium or rubidium is used as a CO₂ carrier. However,since the present invention is characterized in that a hydrophilicpolymer gel membrane that forms a separation-functional membranecontains a CO₂ carrier and a CO₂ hydration catalyst having catalyticactivity at a high temperature of 100° C. or higher, the CO₂ carrier isnot limited to a specific CO₂ carrier as long as it is such a CO₂carrier that a reaction of CO₂ with the CO₂ carrier can be acceleratedby a CO₂ hydration catalyst to attain membrane performance comparable toor higher than the membrane performance (selective permeability of CO₂to hydrogen) shown as an example in the first embodiment.

<2> The above-mentioned embodiments have been described based on theassumption that the CO₂ hydration catalyst contains at least one of atellurous acid compound, a selenious acid compound, an arsenious acidcompound, an orthosilicic acid compound and a molybdic acid compound,but the CO₂ hydration catalyst is not limited to a specific CO₂hydration catalyst as long as it is a CO₂ hydration catalyst which hascatalytic activity for the CO₂ hydration reaction of the above (ChemicalFormula 1) at a high temperature of 100° C. or higher, preferably 130°C. or higher, more preferably 160° C. or higher and which can attainmembrane performance comparable to or higher than the membraneperformance (selective permeability of CO₂ to hydrogen) shown as anexample in the first embodiment when combined with a CO₂ carrier. Whenused in the separation-functional membrane of the present facilitatedtransport membrane, the CO₂ hydration catalyst is preferably one thathas a melting point of 200° C. or higher and is soluble in watersimilarly to the above-mentioned compounds. While the upper limit of therange of temperatures at which the CO₂ hydration catalyst exhibitscatalytic activity is not particularly limited, there is no problem aslong as it is higher than the upper limit of the range of temperaturessuch as the use temperature of the present facilitated transportmembrane in an apparatus including the present facilitated transportmembrane, and the temperature of a source gas supplied to the supplyside surface of the present facilitated transport membrane. Thehydrophilic porous membrane or the like that forms the presentfacilitated transport membrane is also required to have resistance in asimilar temperature range as a matter of course. When the presentfacilitated transport membrane is used at a temperature lower than 100°C., the CO₂ hydration catalyst is not necessarily required to havecatalytic activity at a high temperature of 100° C. or higher.

<3> In the first embodiment, the present facilitated transport membraneis prepared by a method in which a cast solution including an aqueoussolution containing a hydrophilic polymer (PVA/PAA salt copolymer,polyvinyl alcohol (PVA) or the like), a CO₂ carrier and a CO₂ hydrationcatalyst is cast on a hydrophilic PTFE porous membrane, and then gelled,but the present facilitated transport membrane may be prepared by apreparation method other than the above-mentioned preparation method.For example, the present facilitated transport membrane may be preparedby forming a hydrophilic polymer gel membrane that does not contain aCO₂ carrier and a CO₂ hydration catalyst, followed by impregnating thegel membrane with an aqueous solution containing a CO₂ carrier and a CO₂hydration catalyst.

<4> In the first embodiment, the present facilitated transport membranehas a three-layer structure including a hydrophobic PTFE porousmembrane, a separation-functional membrane supported on a hydrophilicPTFE porous membrane and a hydrophobic PTFE porous membrane, but thesupport structure of the present facilitated transport membrane is notlimited to such a three-layer structure. For example, the presentfacilitated transport membrane may have a two-layer structure includinga hydrophobic PTFE porous membrane and a separation-functional membranesupported on a hydrophilic PTFE porous membrane. In the firstembodiment, a case has been described where the separation-functionalmembrane is supported on the hydrophilic PTFE porous membrane, but theseparation-functional membrane may be supported on the hydrophobic PTFEporous membrane.

<5> In the second embodiment, a CO₂ permeable membrane reactor has beendescribed as an application example of the CO₂ separation apparatusincluding the present facilitated transport membrane, but the CO₂separation apparatus including the present facilitated transportmembrane can also be used in a decarbonation step in a hydrogenproduction process other than that in the membrane reactor, and isfurther applicable to processes other than the hydrogen productionprocess, and the CO₂ separation apparatus is not limited to theapplication example shown in the above-mentioned embodiment. The supplyside gas (source gas) supplied to the present facilitated transportmembrane is not limited to the mixed gas shown as an example in theabove-mentioned embodiments.

<6> The mixing ratios of the components in the composition of thepresent facilitated transport membrane, the dimensions of the portionsof the membrane and the like as shown as examples in the above-mentionedembodiments are examples given for easy understanding of the presentinvention, and the present invention is not limited to facilitated CO₂transport membranes having such values.

INDUSTRIAL APPLICABILITY

A facilitated CO₂ transport membrane according to the present inventioncan be used for separating CO₂ from a mixed gas including CO₂ and H₂ ata high selection ratio to hydrogen in a decarbonation step in a hydrogenproduction process, a CO₂ permeable membrane reactor, and so on, and isuseful particularly for separation of CO₂ at a high temperature of 100°C. or higher.

DESCRIPTION OF SYMBOLS

1 separation-functional membrane

2 hydrophilic porous membrane

3, 4 hydrophobic porous membrane

10 CO₂ separation apparatus

11 facilitated CO₂ transport membrane

12 container

12 a, 12 b bottom portion (upper bottom portion and lower bottomportion) of container

13 supply side space

14 permeate side space

15 first feeding port

16 second feeding port

17 first discharge port

18 second discharge port

FG source gas

EG CO₂-separated source gas

PG permeate gas

SG, SG′ sweep gas

The invention claimed is:
 1. A facilitated CO₂ transport membranecomprising a separation-functional membrane that includes a hydrophilicpolymer gel membrane containing a CO₂ carrier and a CO₂ hydrationcatalyst, wherein the CO₂ hydration catalyst is an oxo acid compound ofat least one element selected from group 6 elements and group 14elements.
 2. The facilitated CO₂ transport membrane according to claim1, wherein the CO₂ hydration catalyst has catalytic activity at atemperature of 100° C. or higher.
 3. The facilitated CO₂ transportmembrane according to claim 1, wherein the CO₂ hydration catalyst has amelting point of 200° C. or higher.
 4. The facilitated CO₂ transportmembrane according to claim 1, wherein the CO₂ hydration catalyst issoluble in water.
 5. The facilitated CO₂ transport membrane according toclaim 1, wherein the CO₂ hydration catalyst contains at least anorthosilicic acid compound.
 6. The facilitated CO₂ transport membraneaccording to claim 1, wherein the CO₂ hydration catalyst contains amolybdic acid compound.
 7. The facilitated CO₂ transport membraneaccording to claim 1, wherein the gel membrane is a hydrogel.
 8. Thefacilitated CO₂ transport membrane according to claim 1, wherein the gelmembrane is a polyvinyl alcohol-polyacrylic acid salt copolymer gelmembrane.
 9. The facilitated CO₂ transport membrane according to claim1, wherein the CO₂ carrier contains at least one of a carbonate of analkali metal, a bicarbonate of an alkali metal and a hydroxide of analkali metal.
 10. The facilitated CO₂ transport membrane according toclaim 9, wherein the alkali metal is cesium or rubidium.
 11. Thefacilitated CO₂ transport membrane according to claim 1, wherein theseparation-functional membrane is supported on a hydrophilic porousmembrane.
 12. A method for producing the facilitated CO₂ transportmembrane according to claim 1, the method comprising the steps of:preparing a cast solution including an aqueous solution containing thehydrophilic polymer, the CO₂ carrier and the CO₂ hydration catalyst thatis an oxo acid compound of at least one element selected from group 6elements and group 14 elements and is soluble in water; and casting thecast solution on a hydrophilic porous membrane and then gelling the castsolution to prepare the separation-functional membrane.
 13. A method forseparating CO₂ using a facilitated CO₂ transport membrane, thefacilitated CO₂ transport membrane comprising a separation-functionalmembrane that includes a hydrophilic polymer gel membrane containing aCO₂ carrier with a CO₂ hydration catalyst having catalytic activity at atemperature of 100° C. or higher, wherein a mixed gas containing CO₂ andH₂ and having a temperature of 100° C. or higher is supplied to a supplyside of the facilitated CO₂ transport membrane and the CO₂ passingthrough the facilitated CO₂ transport membrane is separated from themixed gas and, wherein the CO₂ hydration catalyst is an oxo acidcompound of at least one element selected from group 6 elements andgroup 14 elements.
 14. The method of claim 13, wherein the mixed gascontaining CO₂ and H₂ and having a temperature of 100° C. or higher issupplied to a supply side of the facilitated CO₂ transport membraneunder a condition that a pressure difference between the supply side anda permeate side of the facilitated CO₂ transport membrane is not lessthan 200kPa.
 15. A CO₂ separation apparatus comprising a facilitated CO₂transport membrane, the facilitated CO₂ transport membrane comprising aseparation-functional membrane that includes a hydrophilic polymer gelmembrane containing a CO₂ carrier and a CO₂ hydration catalyst havingcatalytic activity at a temperature of 100° C. or higher, wherein theapparatus is configured to supply a mixed gas containing CO₂ and H₂ andhaving a temperature of 100° C. or higher to a supply side of thefacilitated CO₂ transport membrane and the CO₂ passing through thefacilitated CO₂ transport membrane is separated from the mixed gas and,wherein the CO₂ hydration catalyst is an oxo acid compound of at leastone element selected from group 6 elements and group 14 elements. 16.The apparatus of claim 15, wherein the apparatus is configured to supplythe mixed gas containing CO₂ and H₂ and having a temperature of 100° C.or higher is supplied to a supply side of the facilitated CO₂ transportmembrane under a condition that a pressure difference between the supplyside and a permeate side of the facilitated CO₂ transport membrane isnot less than 200kPa.